With the increasing use of minimally invasive surgical techniques in medical diagnosis and therapy, there is a need for a new method of remotely locating and tracking catheters or other medical instruments inside a human or animal body. Currently, x-ray fluoroscopic imaging is the standard catheter tracking technique. However, excessive exposure to x-ray dosages by both the patient and clinician can be harmful. Thus, alternative catheter tracking methods are desirable.
Several alternative methods have been published including some which employ ultrasonic transducers and others which make use of magnetic field measurements.
One known method of catheter location employs one or more magnetic field sources, which are fixed relative to one another and define a spatial reference frame, and one or more magnetic sensors, fixed to the tip of the catheter. The sensors measure the fields produced by the sources, and these measurements are then used to determine the tip's position relative to the reference frame. The same result could alternatively be achieved with the sources replaced by sensors, and the sensors by sources.
This technique relies on accurate prior knowledge of the relative positions of the sources and the spatial forms of their magnetic fields, and of the relative positions and sensitivities of the sensors. Because it is not possible to manufacture sources and sensors with ideal characteristics, purely theoretical calculations of such characteristics are likely to be erroneous, and hence they must be determined from calibration measurements. One advantage of using magnetic fields to track a catheter inside a human or animal body is that the fields are virtually unaffected by the presence of the body. This is due to the very low magnetic susceptibility of body tissue. In contrast, electric and acoustic fields are strongly affected by body tissue. The result is that calibration measurements of a magnetic field tracking system can be made without the presence of the body, before surgery.
A limitation placed on catheters is that they must be small enough in diameter and flexible enough to allow insertion into the relevant part of the body. For example, cardiac catheter diameters should be around 2 mm, and flexible enough to bend to a radius of 10 mm or less. These requirements, and the need to fix the catheter mounted transducers rigidly together, close to the catheter head, demand that these transducers must all be contained in a small volume.
A known catheter tracking method based on the above approach is described in PCT Patent Application WO 96105768 (Ben-Haim et al). In Ben-Haim's method, there are a plurality of magnetic sources, preferably three, and a plurality of catheter mounted sensors, again preferably three. The sensors are preferably wire coils, of the type which measure the local field component parallel to its axis, aligned in orthogonal directions.
Since multiple simultaneous but independent measurements of the magnetic fields are necessary in order to perform a location, the known catheter tracking method requires that the plurality of magnetic sources and the plurality of catheter mounted sensors be arranged independently such that none of their fields can be expressed as a fixed combination of the other fields, and so that none of their measurements can be expressed as a fixed combination of the other measurements. Since a magnetic field is a vector quantity, it is possible for up to three co-located transducers to be mutually independent, provided they are fixed orthogonally with respect to one another. More than three transducers must be spatially separated, in order to be mutually independent.
Known catheter location method suffers from certain disadvantages. Firstly, three magnetic field coils, arranged independently, must be integrated into a small volume near the head of the catheter. This represents a difficult and costly procedure. Secondly, calibration of the sensors in each catheter is a complex procedure, especially measuring their orientations. Factory calibration is thus preferable to calibration of each catheter just prior to use by the medical personnel. However, if the catheters are calibrated in advance, a fool-proof system will be needed to ensure that the correct calibration data for each catheter is entered into the signal processor.
The aforementioned disadvantages associated with calibration, independence of magnetic field transducers, and size of a catheter head, represent technical problems addressed by the catheter tracking system according to the present invention.